Machine felts in which a substrate made up of plastic textile yarns is equipped with a fiber batt layer made of plastic fibers are used in particular in the sector of paper machine belts. Such paper machine belts are used principally as press felts in the press section of a paper machine. The nonwoven fiber layer is manufactured, in principle, in such a way that one or more nonwoven webs are needle felted onto the substrate on one or both sides and are thereby compressed into felts.
Machine felts of this kind are for the most part manufactured in endless fashion. A method suitable for this is evident from EP 0 464 258 A1. In this method the substrate, constituted by a woven or knitted fabric, is built up by winding a substrate web strip whose width is substantially less than the intended width of the substrate, in the form of a helix or screw, onto two spaced-apart rollers until the intended width of the substrate is reached. Simultaneously or subsequently, the substrate is covered with fiber batt webs in the same fashion, and needled to the substrate. The side edges of a felt tube constructed in this fashion are then trimmed, thus producing straight side edges that extend in the machine direction.
With this type of manufacture, because of the winding process the longitudinal yarns proceed at an angle to the machine direction of the machine felt and continuous transverse yarns are not obtained, so that the transverse strength of the machine felt is not very high. To obtain better transverse strength, it has been proposed to connect the edges of the substrate web strips to one another, for example by stitching (U.S. Pat. No. 5,360,656). In the case of substrates manufactured from longitudinal and transverse yarn layers, the edges of the substrate web strips are, according to EP 0 947 623 A1, connected to one another by the fact that the transverse yarns of the transverse yarn layer interengage at the edges, and a connecting yarn is put in place there and welded to the interengaging portions of the transverse yarns. This has the disadvantage, however, that in the region of the edges a strip is produced which, because of the different arrangement and density of the yarns, has different properties (in particular a lower permeability) than the other surfaces of the machine felt. This can result in markings on the paper web.
To remedy this, it has been proposed in EP 1 209 283 A1 to configure the side edges of the substrate web strips in meander fashion with successive projections and indentations, and to abut the substrate web strips against one another in such a way that the projections and indentations interengage, such that the projections completely fill up the indentations. Connection of the edges then takes place via connecting means, for example stitched seams or adhesive tapes. This, too, results in changes in the properties of the completed felt belt in the region of the interengaging edges.
In a further development of the winding method described above, the problem of transverse strength and of increased volume was solved by providing transverse yarns that extend over the entire width of the machine felt (cf. EP 1 837 440 A1). With this method, a first endless substrate module was produced in one or more plies by helically winding on at least one auxiliary substrate web having longitudinal yarns lasered on before, during, or after winding; and onto this substrate module are placed, in one or more plies, substrate module segments that are made up of an auxiliary substrate web and yarn layers lasered thereonto, the substrate module segments being arranged next to one another in such a way that their yarns proceed in a transverse direction. At least one fiber batt web is then needle felted in order to connect the substrate modules and form the fiber batt layer. The function of the respective auxiliary substrate webs is simply to hold the longitudinal and transverse yarns in the intended positions during the manufacturing operation. The auxiliary substrate webs are very largely destroyed upon needle felting of the fiber batt web(s) in order to form the fiber batt layer, and thus become nonfunctional.
Endless machine felts of the kind described above have the disadvantage that installing them, for example, in the press section of a paper machine is difficult. More advantageous in this respect are machine felts that exist at a finite length and comprise at their front ends coupling elements by way of which the ends of the machine felt can be coupled to one another, in the manner of a seam, once in the machine. Such machine felts are therefore also referred to as seam felts.
For the manufacture of a machine felt of this kind, it is proposed in U.S. Pat. No. 5,015,220 firstly to manufacture a felt tube with a peripheral length that corresponds approximately to twice the length of the machine felt to be produced, and with a width that corresponds to the width of the machine felt. The felt tube is made up of a substrate having longitudinal and transverse yarns (such that the longitudinal and transverse yarns can also be present as yarn layers), and of a fiber batt layer that was constituted by needle felting or adhesive bonding of fiber batt webs. The felt tube is then reshaped, by compression transversely to its surface, into a flattened tube whose inner sides rest against one another. The flattened tube then has approximately the length of the machine felt to be produced. At its front ends, transverse yarns and fibers are sufficiently far apart that the longitudinal yarn loops present in these regions are exposed and protrude. At each front end, the respectively pertinent longitudinal yarn loops enclose a channel extending along the front end. When the longitudinal fiber loops of the two front ends are caused to overlap, a common passthrough channel is produced. A coupling wire that connects the two ends of the machine felt to one another can then be inserted into this passthrough channel.
A further development of this machine felt is evident from EP 0 947 627 A1. According to this document, a longitudinal yarn loop is manufactured by helically winding a longitudinal yarn over two spaced-apart rollers, forming two longitudinal yarn layers arranged one above another and forming longitudinal yarn loops at the front ends, and a transverse yarn insert is then slid into the interstice between the longitudinal yarn layers. The transverse ply extends over the width of the longitudinal yarn tube. It prevents the longitudinal yarns from being pressed into one another upon subsequent needle felting of a fiber batt layer, thereby causing yarn displacements that result in a nonuniform properties profile. As in the case of the machine felt according to U.S. Pat. No. 5,015,220, however, the transverse yarns cannot prevent the longitudinal yarns from shifting in the transverse direction upon needling. Their effect is therefore limited, especially since the insertion operation creates increased manufacturing complexity.
The underlying object of the invention is to make available a method for manufacturing a machine felt of finite length having a substrate made of yarn layers, in which displacement of the yarns does not occur especially upon manufacture of the fiber batt layer. A further object is to design a machine felt of finite length in which the yarns of the yarn layer proceed in each case in parallel fashion and largely equidistant from one another.